Micromechanical resonators (or microresonators) are miniature acoustic resonators fabricated using integrated circuit microfabrication techniques. Such microresonators have various uses, such as filters and oscillators. In particular, the resonant frequency can be defined photolithographically, thereby allowing numerous filters spanning from several hundred MHz to several GHz to be realized on a single chip.
Currently, band select filters in cellular handsets are realized using a combination of many dies containing bulk (BAW) or surface (SAW) acoustic wave resonators. The resonant frequency for these BAW resonators is determined generally by the thickness of a deposited thin film and requires a separate film thickness for each filter frequency. This makes integration of multiple frequency filters on a single die both challenging and costly. While in theory SAW resonators can support a wide range of frequencies on a single chip, in practice, the thickness of the metal interdigitated electrodes used to transduce a SAW resonator is varied with frequency, thereby limiting the range of filter bands that can be covered on a single chip.
By basing the resonance on a laterally propagating Lamb wave in a suspended plate with a thickness less than an acoustic wavelength, a wide range of filter frequencies can be achieved on a single wafer by altering the CAD-layout of the devices. Piezoelectric Lamb wave resonators formed in deposited thin films of aluminum nitride, zinc oxide, and lead zirconate titanate, while having much higher coupling coefficients than electrostatically transduced microresonators, still do not have a high enough coupling coefficient for many of the band select filters in wireless handsets. Thus, there is a need for improved microresonators having sufficiently high coupling coefficients and CAD-definable frequencies, as well as new methods for fabricating such microresonators on a single chip.